Modern cathode ray tube systems, for displaying color images for home television, may include an electron gun designed to generate three in-line beams disposed in a common horizontal plane and a self-converging deflection yoke designed to maintain the beams converged as they are scanned over the screen of the tube. In such a system the deflection field of the yoke is inherently astigmatic by design so as to obtain its self-converging characteristic. However, this astigmatism, which desirably produces the self-convergence, at the same time undesirably produces a distortion on the cross-sectional shape of the electron beams. Specifically, the yoke field is over-converging in the vertical plane and under-converging in the horizontal plane. Thus, if the electron gun is arranged to produce a circular beam spot at the center of the screen, the spot will become horizontally elongated with a vertically extending flare or smear when it is scanned to the corners of the screen.
In the in-line self-converging system described above, it has been common practice to provide the electron gun as a unitized structure, that is, one in which the three beams are acted upon by common electrodes which have three separate apertures therein. This type of structure contributes to accuracy and relative rigidity between the electron beam forming structures for each of the three beams. In such guns it is common to provide certain of the electrodes, for example, those between which the main focus lens of the gun is formed, in the form of elongated cups with the electron beam apertures formed in mutually facing floors of the cups. These cups, because of their elongated bathtub-like shape, are often referred to as tubs. These tub electrodes, in addition to having three in-line apertures in the floors of the tubs, also have tubular lips around the apertures which extend into the interiors of the tubs.
I have discovered that the focus fields established between these tub electrodes extend completely through the cylindrical lips surrounding the apertures and that the fringe portions of the three separate fields then merge into a single field which spans the entire length and width of the tub electrodes. These fringe portions of the fields are of much shallower curvature in the elongated direction of the tub electrode than in the orthogonal short direction of the tub. As a result, the fringe field formed in the interior of the tub beyond the ends of the aperture lips is astigmatic, and like that of the self-converging yoke field is over-converging in the vertical plane and under-converging in the horizontal plane, thus further contributing to undesirable beam spot distortion.